One of the major problems facing radio frequency identification (“RFID”) systems today is that of RFID tags not being read, which results in a reduction in tag identification rates. Tags are less likely to be read if a large number of tags is present in a small enclosure. When tags are close to each other, they can shield other tags and mutual coupling between tags can result in tags losing power. Loss of power can result in tags not being identified or taking too long to identify. As a result, some currently available RFID systems have not shown good performance when used with a high density of RFID tags close to each other.
To prevent tags from losing power, persistent state storage bits (“superbits”) that store flags can be implemented in tags. Using such state storage can give substantial improvement in RFID throughput, help eliminate missed tags in passive RFID operation, and improve tag recognition. Using known systems, however, a command sent to a tag to reset a persistent state storage bit might not be received and the capacitor may stay charged. If a particular tag is weakly energized (e.g., if the tag is energized only at one frequency out of 30 frequencies in the band), and if the tag's state storage bit is not reset several times because it misses most reset commands, then this tag could be missed by all identification cycles.
Currently known methods of implementing a state storage bit are based on a high impedance node blocking a capacitor so that the leakage through the high impedance determines the maximum time for which the state storage bit can be stored. Since the high impedance depends on parasitics, the state storage time can vary from a few seconds to a few hours, or even days. Currently known state storage devices allow the capacitor to drain current too quickly or allow the capacitor can retain a charge for too long. In known devices, when the power supply is off, the state storage device dissipates its charge by means of an unknown and widely varying leakage current.